The solvent proton longitudinal (spin-lattice) relaxation time (1/T1M) of a paramagnetic contrast agent is directly related to the image contrast in medical magnetic resonance imaging (MRI). There are two contributors to proton relaxation, known as inner sphere and outer sphere. Solomon and Bloembergen have shown that the inner sphere proton longitudinal relaxation time is dependent upon three factors, namely the rotational correlation time ([unreadable]R), the electronic spin-lattice relaxation time ([unreadable]S), and the lifetime of the water protons in the paramagnetic metal site ([unreadable]M). Outer-sphere relaxation has been found to depend on the correlation time of the solute and the solvent to diffuse apart ([unreadable]) and again the electronic spin-lattice relaxation time. However, direct measurements of these components have not been easy to obtain. Nuclear magnetic relaxation dispersion (NMRD) can only provide estimates to the parameters mentioned above. In this study we report the successful employment of elect ron paramagnetic resonance (EPR), using vanadyl complexed to two different chelates (EDTA and DTPA) as spin probes, in direct measurements of the rotational correlation time. Furthermore, estimates derived from fitting nuclear magnetic relaxation dispersion (NMRD) profiles corroborate the EPR results obtained. This is an ongoing project that has produced important new results and publications in 1997.